In a digital still camera, a movie, etc., an image capturing element such as a CCD (Charge Coupled Device) is used.
Color separation filters having a predetermined repetition pattern for every pixel is attached for coloring to a surface of an image capturing element.
Various types of such color separation filters are known.
One of filter arrangements for widely used color separation filters is a complementary color filter arrangement as that shown in FIG. 2 formed by repetition of two horizontal pixels and four vertical pixels in which a magenta and a green color filters are alternately arranged in this order on a first line along the horizontal direction, a yellow and a cyan color filters are alternately arranged in this order on a second line along the horizontal direction, a green and a magenta color filters are alternately arranged in this order on a third line along the horizontal direction and a yellow and a cyan color filters are alternately arranged in this order on a fourth line along the horizontal direction.
This filter arrangement will be hereinafter referred to as a color difference line sequential arrangement.
A method of generating a luminance signal from an output from an image capturing element comprising color separation filters having the color difference line sequential arrangement may be, in general, a method requiring addition of two vertical pixels×two horizontal pixels (See Japanese Patent Application Laid-Open Gazette No. H8-070466 (page 2, FIG. 4).).
The entire disclosure of Japanese Patent Application Laid-Open Gazette No. H8-070466 is incorporated herein by reference in its entirety.
One of widely used filter arrangements other than the color difference line sequential arrangement is a primary color filter arrangement as that shown in FIG. 10 formed by repetition of two horizontal pixels and two vertical pixels in which color filters for red and green are alternately arranged in this order on a first line along the horizontal direction and color filters for green and blue are alternately arranged in this order on a second line along the horizontal direction.
This filter arrangement will be hereinafter referred to as a primary color Bayer arrangement.
Methods of generating a luminance signal from an output from an image capturing element comprising color filters having the primary color Bayer arrangement include a method requiring addition of two vertical pixels×two horizontal pixels.
Other method of generating a luminance signal from an output from an image capturing element comprising color separation filters having the primary color Bayer arrangement may be a method which requires addition of only green signals within areas consisting of two vertical pixels×two horizontal pixels to thereby generate a high-frequency luminance component (See Japanese Patent Application Laid-Open Gazette No. H8-237672 (page 7, FIGS. 10 through 13).).
The entire disclosure of Japanese Patent Application Laid-Open Gazette No. H8-237672 is incorporated herein by reference in its entirety.
However, where a color separation filter arrangement which is the color difference line sequential arrangement is used, generation of a luminance signal by means of addition of signals representing two vertical pixels×two horizontal pixels gives rise to a problem that a dot-like pseudo signal is generated during image capturing of an object which shows dramatic color changes in the horizontal direction.
For instance, FIG. 6 is a drawing which shows a part of the color filter arrangement which is the color difference line sequential arrangement, and the colors of an object change between the column m and the column m+1.
Ye=Mg=1 and Gr=Cy=0 on the right hand side to the point of change, while Ye=Mg=Gr=Cy=0 on the right hand side to the point of change.
FIG. 7 shows outputs from an image capturing element corresponding to the locations of the respective color filters.
The addition value at two vertical pixels×two horizontal pixels denoted by coordinates (m, n−1), (m+1, n−1), (m, n) and (m+1, n) is two, and the addition value at pixels denoted by coordinates (m, n+1), (m+1, n+1), (m, n+2) and (m+1, n+2) is one.
A luminance signal thus changes even though the object does not show any change in luminance along the vertical direction.
The inventor of the present invention has noticed that this would manifest itself as a dot-like pseudo signal and serve as a factor remarkably degrading the image capturing quality.
In a similar manner, use of color separation filters having the primary color Bayer arrangement leads to a problem that during image capturing of an object whose colors change dramatically in an oblique direction (diagonal direction), a dot-like pseudo signal is generated.
For example, FIG. 11 is a drawing which shows a part of a color filter arrangement which is the primary color Bayer arrangement, and the colors of an object change over a diagonal line which connect the coordinates (m−2, n−2) and (m+3, n+3).
R=Gr=0 and B=1 above and on the right hand side to the point of change, while B=Gr=0 and R=1 below and on the left hand side to the point of change.
FIG. 12 shows outputs from an image capturing element corresponding to the locations of the respective color filters.
The addition value at two vertical pixels×two horizontal pixels denoted by coordinates (m−2, n−2), (m−1, n−2), (m−2, n−1) and (m−1, n−1) is zero, and the addition value at pixels denoted by coordinates (m−1, n−1), (m, n−1) (m−1, n) and (m, n) is two.
A luminance signal thus changes even though the object does not show any change in luminance along the oblique direction.
The inventor of the present invention has noticed that this would manifest itself as a dot-like pseudo signal and serve as a factor remarkably degrading the image capturing quality.
Although a dot-like pseudo signal described above is not generated when a high-frequency luminance signal is generated using only green signals within areas consisting of two vertical pixels×two horizontal pixels, since sampling intervals particularly in an oblique direction become twice as large as those which arise when red, green and blue are all used, the resolution in the oblique direction deteriorates.